Investigations On The Formation Of Secondary Organic Aerosol Products From The Ozonolysis Of Cycloalkenes

Secondary organic aerosol (SOA) has been an issue of enormous interest in recent years because of the potential impacts of SOA on climate change, visibility degradation, health effects and tropospheric chemistry. Terpenes are important precursors of SOA on regional and global scales. In this study, we carried out research on the 1-methylcyclohexene/ozone and methylenecyclohexane/ozone reactions as simpler model systems forα-andβ-pinene, respectively. Multifunctional organic acids are important components of SOA and are thought to be the key species in gas-to-particle conversion, and thus these products are the focus of this study. The preliminary work was carried out using the ozonolysis of cyclohexene as a model and the experimental set-up and methodology for studies of condensable ozonolysis products were evaluated and improved. The formation of secondary organic aerosol products from the gas-phase ozonolysis of 1-methylcyclohexene and methylenecyclohexane was then investigated in smog chamber experiments. Using BF3/MeOH derivatization and GC-MS analysis, the low-volatility products formed in the reactions were identified and quantified. Further experiments were carried out using different OH radical scavengers and relative humidities in order to investigate the impacts of [RO2]/[HO2] ratios and water molecules on product formation. The mechanisms of formation of major condensable products were analyzed and differences and similarities between the endo-and exo-cyclic systems were discussed.The experimental results show that for the ozonolysis of 1-methylcyclohexene, C5-C7 multifunctional acids were major acidic products in the condensed phase. C5 and C6 organic acids all derive from the unimolecular decomposition of the Criegee intermediate and their formation mechanisms involve one or more RO2→RO reaction steps; while C7 acid likely derive from the direct rearrangement of the CI. C5 and C6 diacids and aldehydic acids are major condensable products from the ozonolysis of methylenecyclohexane and their yields were in good correlation with the same products from 1-methylcyclohexene, indicating similar reaction mechanisms between the endo- and cyclic systems. On the basis of experimental findings, detailed gas-phase mechanisms were developed to explain the observed product formation.